Fix missing paper-behaviors.html when importing paper-elements.html

[baxy]

Reference: https://github.com/PolymerElements/paper-elements/issues/54

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[baxy]

I signed the CLA.

[googlebot]

CLAs look good, thanks!

[addyosmani]

Afaik, this was fixed in https://github.com/PolymerElements/paper-elements/issues/58 with the preference of addressing this issue in the paper-elements repo vs in the behaviours repo. Thanks for your pull request, regardless <3